Electron multiplier



Jan. 11, 1944. :F. .1. s. VAN DEN BOSCH 5 ELECTRON MUL'T IBLI ER "FiledJune 16, 1941 Patented Jan. 11, 1944 ELECTRON MULTIPLIER Francois JosephGerard van den Bosch, East Groydon, England, assignor to Vacuum-ScienceProducts Limited, London, England, a British company Application June16, 1941, Serial No. 398,341 In Great Britain February 23, 1940 3Claims.

This invention relates to multiplier apparatus and is concerned withsuch apparatus of the kind comprising an electron multiplier having aseries of electrodes comprising a plurality of secondary cathodes and acollector electrode, and means for applying to this series of electrodesprogres sively increasing positive potentials. The aforesaid series ofelectrodes are enclosed in an enve lope together with a primary cathode,which may be thermionic or photo-electric, and usually a control gridand an accelerator electrode for the primary emission. Electrons emittedby the primary cathode are caused to impinge on the first of the seriesof secondary cathodes and liberate other electrons which impinge on thenext secondary cathode, and so on, in succession to the collector.

Usually, the series of electrodes comprising the secondary cathodes andcollector are uniformly separated one electrode from another and thepotentials applied to these electrodes are such that the potentialdifference between each secondary cathode and the next followingelectrode is of uniform value, for instance 300 volts.

In general it is of advantage to have adequate separation of thesecondary cathodes so as to permit requisite acceleration of theelectrons from one secondary cathode to the next following electrode. Itis found, however, that there is in some cases a falling oiT in theresponse of the electron multiplication as regards the later multiplierstages, and an object of the invention is to improve the operation ofthe multiplier in this respect.

This invention comprises an electron multiplier apparatus of the kindspecified wherein the potentials applied to at least some of the saidseries of electrodes and/or the spacings of these electrodes are soproportioned that the rate of increase of the field strength along thepath of electrons is progressively increased.

The invention further comprises an electron multiplier apparatus of thekind specified wherein the electric field between alternate electrodesof the series has a zone located in front of the intermediate electrodecorresponding to the potential applied to the intermediate electrode.

The spacing of adjacent electrodes of the series decreases towards thecollector electrode. Furthermore, the electron multiplier may beconstructed with the spacing of adjacent electrodes of the seriesdecreasing towards the collector electrode and operated with thepotential difference of adjacent electrodes of the series increasingtowards the collector electrode.

It has been found that a falling off in the response of an electronmultiplier when operating with high frequencies can be attributed to theelectron transit time through the multiplier. The transit time betweensuccessive secondary cathodes is greatest for the later stages of themultiplier, due to the density of electrons passing between cathodes atthe end of the series of electrodes being greater than that of electronspassing between cathodes at the beginning of the series. The increasingrate of the rise of applied potentials and/or the relatively closespacing of the secondary cathodes, particularly at the output end of theseries, considerably improves the response of the multiplier withoutadversely affecting the multiplication per stage. Also, secondaryelectrons emitted by a secondary cathode are influenced by the strongfield obtained by the increased potential applied to the followingelectrode and/ or by the relatively close proximity of the followingelectrode so that these second ary electrons are more completelyaccelerated towards this following electrode. The multiplicationobtained for the later stages of the electron multiplier is thusmaterially improved. The transit time between secondary cathodes at thebeginning of the series is not of great importance and wide spacing ofthese secondary cathodes has the advantage that it permits adequateaccelerations of the electrons to be obtained.

The invention also comprises an electron multiplier apparatus asaforesaid wherein at least some of the secondary cathodes consist ofgrid, perforated plate or like apertured members and wherein the numberof apertures in the secondary cathodes increases for successivesecondary cathodes towards the collector electrode.

Specific embodiments of the invention are illustrated by way of examplesin the accompanying drawing, in which:

Figures 1, 2 and 3 are diagrams showing different forms of multiplierapparatus according to the invention.

In Figure 1 there is shown an electron multiplier ii! comprising athermionic primary cathode ii together with a control grid i2 and anaccelerator 13 for the primary electrons. In addition, there is aplurality of secondary cathodes id each in the form of a grid,perforated plate or like apertured member, a final secondary cathode itin the form of an imperforate plate and a grid-like collector it.Positive potentials with respect to the cathode H are applied to theaccelerator, the secondary cathodes and the collector from appropriatetappings on a potential divider ll, across which an electric supplyindicated at it! is connected. A signal input device [9 is connected tothe control grid 52 and the cathode i l and an output device it isconnected the collector electrode Hi. The spacing between successiveelectrodes of the series consisting of the secondary cathodes andcollector electrode progressively decreases. Thus, the spacing of thefirst two secondary cathodes id is greater than that between the secondand third secondary cathodes it. Similarly, the spacing between thesecond and third secondary cathodes it is greater than that between thethird and fourth secondary cathodes l4, and so on, to the collector I5.Uniformly increasing potentials are applied to the series of electrodescomprising the secondary cathodes and collector electrode from thepotential divider ll, for example a potential diiference of 300 volts.

In the apparatus shown in Figure 2 the secondary cathodes it and it areequally spaced and the potentials applied to these secondary cathodesfrom the potential divider H instead of being of uniformly increasingvalue the rate of increase of these potentials also increases. thepotential difference of the first two secondary cathodes H is less thanthe potential difference across the second and third secondary cathodesand this latter potential difference is again less than the potentialdifference across the third and fourth secondary cathodes, and so on.For example potentials applied to the secondary cathodes and collectormay progressively increase by 390 volts plus an additional per cent ofthe total voltage value thus obtained, although good results have beenobtained by employing a potential difference between successive electrodes which rises by 50 volts for each stage.

In the arrangement shown in Figure 3 the secondary cathodes andcollector are difierently spaced in the manner shown in Figure 1 andhave an increasing potential difference between successive secondarycathodes according to the arrangement shown in Figure 2. By arrangingalternate electrodes of the series of secondary electrodes and collectorin any of the ways described with reference to the drawing there isobtained a potential Zone of the same value as the potential applied tothe intermediate secondary cathode which lies to the front of thissecondary cathode. In this way secondary electrons emittted by thisintermediate electrode are subjected to a strong field of the followingelectrode penetrating the apertures of the electrode from which thesecondary electrons are emitted and serving to accelerate theseelectrons in greaternumber towards the following electrode than is thecase with the normal arrangement of equally spaced secondary cathodesoperated with a uniform potential gradient.

The invention employs apertured secondary cathodes It in which the totalnumber of apertures increases for the successive cathodes in thedirection towards the final collector. In a construction in which thesecondary cathodes consist of wire grids or meshes the grid or mesh maybe finer for the cathodes at the end of the series than for the cathodesat the beginning of the series; for instance, the grid or mesh of thecathodes may be progressively finer throughout the series. In the caseof secondary cathodes formed of perforated plates the number ofperforations may be increased, for instance, progressively for theseveral secondary cathodes. In the case where the secondary cathodesconsist of perforated plates with apertures tapering in cross-section,as described in the specification of my United States Patent No,2,254,128, these apertures may be formed of less depth for secondarycathodes at the end of the series than for secondary cathodes at thebeginning of the series, so as to enable close spacing of the secondarycathodes at the end of the series. Auxiliary electrodes may be providedfor each of the secondary cathodes and in front thereof, as described inthe specification of my Unitedstates Patent No. 2,285,848.

I claim:

1. An electron multiplier apparatus comprising an electron dischargetube having a primary cathode and a series of electrodes consisting of aplurality of secondary cathodes and a collector in said tube, at leastsome of said secondary cathodes having multiple apertures for thepassage of electrons and the number of apertures in the secondarycathodes increasing for successive secondary cathodes towards thecollector, and means for applying to at least some of said series ofelectrodes positive potentials with respect to the primary cathode ofprogressively increasing value so that the potential differences betweensucceeding pairs of electrodes increase, the spacing of said series ofelectrodes decreasing towards the collector and the rate of increase ofthe field strength along the path of electrons between successiveelectrodes being progressively increased.

2. An electron multiplier apparatus comprising an electron dischargetube having a primary cathode and a series of electrodes consisting of aplurality of secondary cathodes and a collector in said tube, at leastsome of said secondary cathodes having multiple apertures for thepassage of electrons and the number of apertures in the secondarycathodes increasing for successive secondary cathodes towards thecollector and the spacing of at least some of said series of electrodesdecreasing towards the collector, and means for applying to said seriesof electrodes substantially uniformly increasing positive potentialswith respect to the primary cathode.

3. An electron multiplier apparatus comprising an electron dischargetube having a primary cathode and a substantially uniformly spacedseries of electrodes consisting of a plurality of secondary cathodes anda collector in sai tube, at least some of said secondary cathodes havingmultiple apertures for the passage of electrons and the number ofapertures in the secondary cathodes increasing for successive secondarycathodes towards the collector, and means for applying to said series ofelectrodes, positive potentials which are of progressively increasingvalue towards the collector and which provide potential differenceswhich increase between successive pairs of electrodes towards thecollector, the spacing of at least some of said series of electrodesdecreasing towards the collector.

FRANCOIS JOSEPH GERARD VAN DEN BOSCH.

